The present disclosure relates to nitride semiconductor devices, and more particularly to nitride semiconductor devices including electrode pads above an active region.
Group III-V nitride semiconductor (hereinafter referred to as nitride semiconductor) is a compound of group III elements such as gallium (Ga), aluminum (Al), and indium (In), and a group V element, nitrogen (N), and forms an alloy represented by the general formula AlxGa1-x-yInyN, where 0≦x≦1, 0≦y≦1, and x+y≦1. Nitride semiconductor has a wide band gap and a direct transition band structure, and is thus applied to short-wavelength optical devices. Applying nitride semiconductor to electronic devices is also considered, since it has a high breakdown voltage and high electron saturation velocity as its characteristics. In particular, heterojunction field effect transistors (hereinafter referred to as HFETs) utilizing two-dimensional electron gas (hereinafter referred to as 2DEG) generated at the interface between an AlxGa1-xN layer, where 0<x≦1, and a GaN layer, which are sequentially formed by epitaxial growth on a semi-insulating substrate, are being developed as high output devices and high frequency devices.
In HFETs, not only electrons are supplied from an AlGaN Schottky layer, which is a carrier supply layer, but also charges are supplied by a polarization effect caused by spontaneous polarization and piezoelectric polarization. HFETs made of nitride semiconductor have an electron density higher than 1013 cm−2, which is higher than HFETs made of aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs) by about one digit. As such, HFETs made of nitride semiconductor are expected to have a higher drain current density than HFETs made of GaAs. An element having a maximum drain current larger than 1 A/mm is reported. (See, for example, Yuji Ando, Yasuhiro Okamoto, Hironobu Miyamoto, Tatsuo Nakayama, Takashi Inoue, Masaaki Kuzuhara, Evaluation of High Breakdown Voltage AlGaN/GaN Heterojunction FET, IEICE Technical Report, ED2002-214, CPM2002-105 (2002-10), p. 29-34.) Furthermore, nitride semiconductor has a wide band gap (for example, GaN has a band gap of 3.4 eV) to exhibit high breakdown voltage characteristics. HFETs made of nitride semiconductor has a breakdown voltage of 100 V or more between a gate electrode and a drain electrode. Therefore, applying electronic devices made of nitride semiconductor such as HFETs as high frequency elements and elements handling greater power with a smaller size than conventional devices is considered.
Nitride semiconductor devices have active regions of about 1/3-1/10 the sizes of active regions in silicon (Si) semiconductor devices. However, when a nitride semiconductor device is used as a power device in which a large current flows, the sizes of wires and ribbons coupled to electrode pads are preferably large. Even if an active region is reduced, it is less effective in reducing the size of the nitride semiconductor device. In order to reduce the size of a nitride semiconductor device, what is called a pad-on-element structure in which electrode pads are formed on an active region is considered. (See, for example, Japanese Patent Publication No. 2008-177527.)